In adition to the links provided

Ping.

ping alows us to test the conectivity of a networked computer, this alows us to also spot any delays between the two computers. Ping can also alow us to test if our dns is working corectly by testing ping yahoo.com for example we can test to see if our computer is able to make the conection and this tells us if we can recieve domain name lookups.

Netstat

netstat is a good tool to view active conections on your computer, this is usefull for sometimes spotting backdoors, or to see if a server is running when troubleshooting.

tracert

tracert can tell us the path it taks to get from computer A to computer B, which can also alow us to spot slow areas, and can be used to analize other things.